How can animated shape keys be used in the NLA when a character is linked from another .blend?
I created a .blend file start with the default cube.  I gave it a relative shape key and shrunk one of the faces.  I then added keyframes at 1 and 31 with influence 0 and 1.0 respectively.  That makes the face shrink over time.  

I then duplicated the cube with alt-d.  Since both cubes have the same mesh, the action is on both of them.  I made a single-user copy of the mesh by clicking the [2] button next to the mesh's name.

It still had a link to the action datablock.  While there are probably easier ways to do this I went into the NLA editor and used the vv icon to move the action into a track, then I deleted the track.  

Now I have one cube with the action, and another cube with no action.  I think I am at the point where I can answer your question.

I took the cube that has no action and in the NLA editor I used the menu Add > Add Track to put an NLA track on it.  Then I use Add > Add Action Strip and select the KeyAction.  This causes the action to be added as a strip in the track starting at the current frame of the timeline (I had my time cursor at 31).

Now I have two cubes, each with its own mesh datablock (although they have identical shape keys) and those shape keys are animated by the same action, but the action occurs at different times, because the strip is offsetted.

Although this may not be exactly what you want to accomplish, maybe it shines light on the relationships between the datablocks and guides you to your answer.